The hypothesis driving the current proposal is that multiple y-secretase substrates are misprocessed in mutant presenilin 1 familial Alzheimer's disease (PS1 FAD) and that a multisubstrate misprocessing signature is also similarly associated with sporadic AD. To this end, we have recently demonstrated that APP p3-like alcadein peptides (p3-Alcs) are generated by typical a- and y-secretases, and p3-Alc C termini are modulated by FAD mutant PS1. In neurons. Ale proteins form complexes with X11L molecules, which, in turn, complex with APP. For this reason, we chose Ale proteins as representative y-secretase substrates that might undergo misprocessing in AD in parallel with APP. Plotting minor/major p3-Alc variant ratios (analogous to AB42/40 ratios) against the corresponding AB42/40 ratios in media conditioned by a panel of FAD-mutant PSI-expressing cells reveals a highly linear covariant relationship between p3-Alc ratios and AB42/40 ratios. We interpret the disease-related change in regression slope as a signature for y secretase dysfunction. p3-Alcs were also detected in human cerebrospinal fluid (CSF), and, again, a covariant signature was associated with the AD phenotype. The association of the sporadic AD phenotype with a distinct p3-Alc:Ap covariant ratio signature provides evidence that y-secretase dysfunction may contribute to the pathogenesis of sporadic AD. Thus, our Specific Aims are: 1) To characterize Ale metabolites in regions of the cerebral cortex of elderly non-demented humans and patients with differential degrees of dementia; 2) To characterize the distribution of Ales in the cerebral cortex of elderly nondemented humans and patients with differential degrees of dementia; and 3) To characterize the cortical and synaptic distribution of Ales in relevant mouse models of AD and analyze their relationship with the progression of neuronal alterations. The information from human brain and from the brains of mouse models should guide us in future efforts to understand the AB:Alc metabolic covariance as well as the possible implications of AB:Alc metabolic covariance for the pathogenesis and diagnosis of sporadic AD.